1. Field of the Invention
The present invention generally relates to spatial diversity radio receiving systems and, more particularly, to a Digital Post-Detection FM spatial diversity combination circuit for fading channels typical of cellular radio operating environments.
2. Description of the Prior Art
Antenna diversity, also known as spatial diversity, is a useful technique for combating Rayleigh fading. The instantaneous fading characteristics seen by two or more receivers operating off of physically separated receive antennas will be different. If the antennas are separated by an appropriate distance, the fading characteristics which will be seen by the two receivers will be largely uncorrelated. This means that it is highly likely that the signal quality of one of the receivers will be better than the signal quality of the other receiver at any given time. By adaptively using the output from the receiver with the better signal quality, the overall output signal to noise ratio (SNR) can be improved.
The key to effectively implementing diversity combining is determining which receiver has the better signal quality. In the past, fixed analog combining techniques were used. These analog combining methods would be optimized for certain operating environments such as either a thermal noise limited environment or a carrier to interference limited environment. A problem with the analog approach is that the analog cellular channel is very dynamic and its properties can change from one type of environment to another. This digital FM spatial combining technique can adapt to accommodate the environment. Analog based diversity combining systems are typically designed as either a maximal ratio combiner, equal gain combiner or a switching combiner.